Size Matters

How much are you really eating? Probably more than you think. Here's how to judge for yourself.

While running is one of the most effective activities for burning calories, it's still a lot easier to eat–or not eat–25 calories (a Hershey's Kiss) than it is to run them off (at least a quarter mile). And that's why runners who spend months training for a marathon might still be carrying a few extra pounds as they cross the finish line. "There's also a big variable in the energy cost of running–which differs between genders, ethnicities, age, weight–which can result in 10 to 15 percent variances in how much weight two different runners might lose," says Ralph La Forge, M.Sc., a physiologist at Duke University. If you've always relied on the notion that 35 miles of running will burn exactly 3,500 calories and, therefore, result in the loss of one pound of body weight, you're missing out on an important part of the equation.

As La Forge explains, that 100-calories-per-mile standard (which is built into the readouts on your gym's treadmills) is the gross energy cost of running–strictly how many calories you burn going from point A to point B. But runners who are trying to lose weight need to also know the net energy cost of their runs so that they can more accurately calculate their entire calorie burn for the day. For example, if you go for a three-mile run and it takes a half hour, you need to subtract out however many calories you would have burned during those 30 minutes even if you'd been just sitting at your desk (about 80 calories). That means the true net energy cost of your run over the course of the day is more like 220 calories than 300.

It's also important for weight-conscious runners to factor in what La Forge calls "energy conservation." "About a third of us wipe out almost half of the energy expenditure of a workout by conserving energy afterward," he says. If your long run leaves you so exhausted that you spend the rest of your Saturday on the couch watching movies rather than cleaning the house or working in the garden, you may end up burning fewer calories throughout the day than if you did a slightly shorter run but still had the energy to take the kids to the park in the afternoon for batting practice. And, of course, you have to beware of "energy compensation"–better known as eating more calories after a workout than you burned off during the run. "I met a guy who ran purely because he liked ice cream," says Wansink. "But to equal his daily pint, he needed to run 12 miles every day." Most of us don't (or can't) do that.

"If you're exercising primarily for health, none of this matters that much," says La Forge, who takes note of the many benefits of exercise, including its link to a reduced risk of chronic diseases such as diabetes and heart disease. But if you're running to lose weight, you have to keep in mind all the variables. "I believe in exercise," says Wansink. "But I also know how quickly you can eat many more calories than you will run off in a normal workout."